Bidirectional converter with preferential direction and reactive power-capable inverter having said converter

ABSTRACT

A half-bridge of a bidirectional converter is divided into a first and a second conduction path connected in parallel. In each of the conduction paths a switching element and a freewheeling diode are connected in series, and the center points of the conduction paths are connected via a second inductor. The second inductor is connected in series with a first inductor which is connected to the center point of the second conduction path. The half-bridge has two operating modes. In each of the two operating modes the switching element in one of the two conduction paths is clocked at a high frequency to cause a flow of energy in one of two directions between a pair of high voltage-side connections and a pair of low voltage-side connections to the half-bridge. The two switching elements are of different types, the switching element in the first conduction path causing higher switching losses.

REFERENCES TO RELATED APPLICATION

The application claims priority to German Patent Application No. 10 2013110 507.6, entitled, “Bidirektionaler Wandler mit Vorzugsrichtung andblindleistungsfähige Wechselrichter mit diesem Wandler,” filed on Sep.23, 2013.

FIELD

The disclosure relates to a bidirectional converter. More particular,the disclosure relates to a bidirectional converter that can be used asa bidirectional DC-to-DC converter, for example, between two DC-voltageintermediate circuits. The disclosure also relates to reactivepower-capable inverters which have at least one bidirectional converteras a sinusoidal half wave-forming part. Inverters of this type arecapable of providing not only active power but also reactive power,owing to the bidirectional design of the converter.

BACKGROUND

It is known, in principle, that a step-up converter and a step-downconverter can be combined between two DC-voltage intermediate circuitsto form a half-bridge, which is connected between the poles of theDC-voltage intermediate circuit for a higher intermediate circuitvoltage and the center point of which is connected via an inductor to apole of the DC-voltage intermediate circuit for the lower intermediatecircuit voltage. In the case of such a half-bridge, the step-downconverter freewheeling diode is connected in parallel with the step-upconverter switch and the step-up converter freewheeling diode isconnected in parallel with the step-down converter switch. In order tobe able to keep the inductance and hence the installation size of theinductor connected to the center point of the half-bridge, the switch inthe respective step-up converter operation or step-down converteroperation of the half-bridge must be clocked at a high frequency(usually several ten kHz). In order to realize this with low switchinglosses, MOSFET semiconductor switches, which cause comparatively lowswitching losses when clocked at high frequency, are preferred. However,MOSFET semiconductor switches have parasitic internal diodes which arereferred to as body diodes. These are connected in parallel with thefreewheeling diodes of the semiconductor bridge and can therefore, inprinciple, become conducting under the same boundary conditions as thefreewheeling diodes. However, they are not suitable for rapid switchingand, in the case of rapid switching, would cause high switching losses,poor EMC behaviour and, in some circumstances, would even cause thecircuit to be destroyed.

A reactive power-capable inverter having two parallel half-bridgesconnected between a first connection and a second connection is knownfrom U.S. Pat. No. 6,847,196 B2. Each half-bridge has two parallelconduction paths. In each of the conduction paths a MOSFET as aswitching element is connected in series with a freewheeling diode. Oneswitching element in one of the conduction paths and one freewheelingdiode in the other of the conduction paths are connected to the firstand to the second connection, respectively. The center points of the twoconduction paths are connected to one another via two small inductors.The connection point of the two inductors is connected via a furtherinductor to one alternating-current connection. The otheralternating-current connection is connected to a correspondingconnection point of the other half-bridge. By alternating operation ofthe two half-bridges as step-down converters, successive half-waves ofan alternating current are formed, which is output at thealternating-current connections. The respective non-current-forminghalf-bridge is switched through. The alternatively possible step-upconverter operation of the half-bridges makes it possible to reverse theflow of energy and hence, for example, also to output reactive power atthe alternating-current connections. During operation of the knowninverter, commutation of the current from the respectively clockedMOSFET to the body diode of the MOSFET in the other conduction branch ofthe same half-bridge is prevented by the inductive voltage division ofthe inductors, with the result that the current exclusively commutatesonto the freewheeling diode connected in series with the clocked MOSFETin the same conduction path. The known inverter is comparativelyelaborate owing to three inductors in total per half-bridge. It also hasa total of four comparatively expensive MOSFETs.

A reactive power-capable inverter is known from U.S. 2011/0013438 A1, inthe case of which both MOSFET semiconductor switches and IGBTsemiconductor switches are used. The known inverter is of the NPC(neutral point clamped) circuit type with freewheeling diodes foroutputting reactive power. U.S. 2011/0013438 A1 also describes circuitshaving half-bridges, in the case of which conduction paths populatedwith MOSFET semiconductor switches are decoupled by inductors. In thecase of inverters with NPC circuit which have no more than two MOSFETsemiconductor switches and two IGBT semiconductor switches, no inductorswhich decouple any conduction paths are provided, however.

The combination of a step-down converter circuit and a commutator toform a photovoltaic inverter is known from EP 2 421 135 A2. Thestep-down converter circuit forms an incoming direct current inhalf-waves, which are converted into an alternating current by means ofthe commutator circuit. The step-down converter circuit has two partialstep-down converters which are designed and arranged so as to bemirror-symmetrical about a center point apart from the forwarddirections of their diodes and the blocking directions of theirswitches. Even if the step-down converter circuit enables a phase shiftof the current with respect to the voltage in each half-wave, it is notunconditionally reactive power-capable. It does not enable differentmathematical signs for current and voltage at the alternating-currentoutput of the photovoltaic inverter and is therefore only able to outputreactive power at a high distortion factor.

A circuit is known from U.S. Pat. No. 5,107,151 A, in the case of whichtwo inputs are connected via a half-bridge to an output. The half-bridgehas two conduction paths between the two inputs. In each of theconduction paths one switching element is connected in series with afreewheeling diode. In this case, the switching element in the oneconduction path is connected to the one input and the switching elementin the other conduction path is connected to the other input. Theopposite applies in the case of the freewheeling diodes. Center pointsof the conduction paths are connected to one another via an inductor.The center point of the second conduction path is connected via afurther inductor to an output of the circuit. Two of such circuits canbe combined to form a full-bridge in order to drive an electric motor,wherein only one further inductor is provided in addition to theinductors between the center points of the conduction paths of the twobridges. During operation of the known circuit, in the case of eachhalf-bridge only one of the switching elements, which is used forpulse-width modulation, is clocked at a high frequency, while the otherswitching element, which is used for commutation, is clocked at a lowfrequency. In the case of the different clock rates of the switchingelements, the switching element which is clocked at a low frequency isdesigned as an IGBT, in contrast to the switching element which isclocked at a high frequency, which is a MOSFET. U.S. Pat. No. 5,107,151A does not describe a bidirectional converter or bidirectional inverterhaving switching elements of different design in the half-bridges.

Thus, there is still a need for a bidirectional converter and reactivepower-capable inverters based thereon, which have a simple andinexpensive design while still having a high practical efficiency.

SUMMARY

In one aspect, the present disclosure provides a bidirectional convertercomprising a first connection, a second connection, a third connectionand a fourth connection, wherein the first and the second connectionform a pair of high voltage-side connections between which a firstvoltage (U_high) is present during operation of the converter, andwherein the third and the fourth connection form a pair of lowvoltage-side connections between which a second voltage (U_low) ispresent during operation of the converter. The first voltage (U_high) isat least as high as the second voltage (U_low). The bidirectionalconverter further comprises a half-bridge, which comprises two switchingelements and two freewheeling diodes, wherein the half-bridge is dividedinto a first conduction path and a second conduction path, which areconnected in parallel between the first connection and the secondconnection. In each of the first and second conduction paths, one of theswitching elements and one of the freewheeling diodes are connected inseries, and center points of the first and second conduction paths areconnected to one another via a smaller inductor. The switching elementin the first conduction path is connected to another connection of thepair of high voltage-side connections in the second conduction path, andthe freewheeling diode in the first conduction path is connected toanother connection of the pair of high voltage-side connections in thesecond conduction path. The half-bridge has a first operating mode, inwhich the switching element in the first conduction path is clocked at ahigh frequency in order to cause a flow of energy in a first directionbetween the pair of high voltage-side connections and the pair of lowvoltage-side connections. The half-bridge has a second operating mode,in which it drives the switching element in the second conduction pathat a high frequency in order to cause a flow of energy in a seconddirection which is opposite to the first direction between the pair ofhigh voltage-side connections and the pair of low voltage-sideconnections. The switching element in the second conduction path has abody diode. The bidirectional converter further comprises a largerinductor, which is connected between the center point of the secondconduction path and the third connection, wherein the smaller inductorand the larger inductor are connected in series between the center pointof the first conduction path and the third connection, while only thelarger inductor is connected between the center point of the secondconduction path and the third connection. In the bidirectionalconverter, the two switching elements are of different types, whereinthe switching element in the first conduction path does not have a bodydiode and causes higher switching losses than the switching element inthe second conduction path. The second direction may be a preferreddirection of the flow of energy between the pair of high voltage-sideconnections and the pair of low voltage-side connections.

In a further aspect, the present disclosure provides an inverterincluding the converter as defined above, and a commutator. One side ofthe commutator is connected to the third connection and the fourthconnection of the half-bridge, and the other side of the commutator isconfigured to be connected to an alternating power grid.

In a further aspect, the present disclosure provides an inverterincluding the converter as defined above and further comprising afurther half-bridge, wherein the half-bridge and the further half-bridgeare designed and arranged so as to be mirror-symmetrical about a commonconnection of the common second connection and the common fourthconnection of the two half-bridges except for the forward directions oftheir diodes and the blocking directions of their switching elements.The inverter further includes a commutator, one side of the commutatorbeing connected to the third connections of the half-bridge and thefurther half-bridge, and the other side of the commutator beingconfigured to be connected to an alternating power grid.

In a further aspect, the present disclosure provides an inverterincluding the converter as defined above and a further bidirectionalconverter, identical to the converter and provided between the firstconnection and the second connection on one side, and a fifth connectionand a sixth connection on another side. An alternating power grid isconnectable to the third connection and the fifth connection, which areconnected via the larger inductors of the converter and the furtherconverter to the center points of the second conduction paths of the twohalf-bridges of the converter and the further converter. The fourthconnection and the sixth connection are connected to one another.

Other features and advantages of the present disclosure will becomeapparent to one skilled in the art upon examination of the followingdrawings and the detailed description. It is intended that all suchadditional features and advantages be included herein within the scopeof the present disclosure, as defined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be better understood with reference to the followingdrawings. The components in the drawings are not necessarily to scale,emphasis instead being placed upon clearly illustrating the principlesof the present disclosure. In the drawings, like reference numeralsdesignate corresponding parts throughout the several views.

FIG. 1 shows a bidirectional converter according to the disclosurehaving a preferred direction of the flow of energy from an intermediatecircuit with lower voltage to an intermediate circuit with highervoltage according to one embodiment.

FIG. 2 shows separately the parts of the converter according to FIG. 1which are active during its preferred operation as step-up converter.

FIG. 3 shows the parts of the converter according to FIG. 1 which areactive during its less preferred operation as step-down converteraccording to one embodiment.

FIG. 4 shows a converter according to the disclosure, in which thepreferred direction of the flow of energy runs from an intermediatecircuit with higher voltage to an intermediate circuit with lowervoltage according to one embodiment.

FIG. 5 shows a bidirectional inverter according to the disclosure havinga converter according to FIG. 4 and an output-side commutator.

FIG. 6 shows a bidirectional inverter according to the disclosure havinga converter which has two subcircuits in mirror-symmetrical arrangementcorresponding to FIG. 4 and an output-side commutator.

FIG. 7 shows a reactive power-capable inverter according to thedisclosure having two bidirectional converters according to thedisclosure operated in half-waves, which converters likewisesubstantially correspond to FIG. 4.

DETAILED DESCRIPTION

A bidirectional converter according to the disclosure has a firstconnection, a second connection, a third connection and a fourthconnection, wherein the first and the second connection form a pair ofhigh voltage-side connections between which a first voltage is presentduring operation of the converter, and wherein the third and the fourthconnection form a pair of low voltage-side connections between which asecond voltage is present during operation of the converter, wherein thefirst voltage is at least as high as the second voltage. A bidirectionalconverter according to the disclosure also has a half-bridge, whichcomprises two switching elements and two freewheeling diodes, whereinthe half-bridge is divided into a first conduction path and a secondconduction path, which are connected in parallel between the firstconnection and the second connection. In each of the conduction pathsone of the switching elements and one of the freewheeling diodes areconnected in series and the center points of which conduction paths areconnected to one another via a smaller inductor. The two switchingelements and the two freewheeling diodes are divided in diagonalarrangement between the two conduction paths, that is to say that boththe two switching elements and the two freewheeling diodes in the twoconduction paths are connected to different connections of the pair ofhigh voltage-side connections. In other words, if the one switchingelement is directly connected to the first connection, the otherswitching element is directly connected to the second connection and, inthe case of the freewheeling diodes, precisely the reverse is true.

The half-bridge has a first operating mode, in which the switchingelement in the first conduction path is clocked at a high frequency inorder to cause a flow of energy in a first direction between the pair ofhigh voltage-side connections and the pair of low voltage-sideconnections. The half-bridge also has a second operating mode, in whichit drives the switching element in the second conduction path at a highfrequency in order to cause a flow of energy in a second direction whichis opposite to the first direction between the pair of high voltage-sideconnections and the pair of low voltage-side connections. That is to saythat, in the one direction between the pair of high voltage-sideconnections and the pair of low voltage-side connections, the converteracts as a step-down converter and, in the opposite direction, it acts asa step-up converter. The high-frequency clocking of the respectiveswitching element usually takes place at a frequency from a few hundredto several 10 kHz, for example at 16 kHz, wherein a current flowing viathe bidirectional converter in the respective direction between the pairof high voltage-side connections and the pair of low voltage-sideconnections can be formed by pulse-width modulation.

The bidirectional converter according to the disclosure additionally hasa larger inductor, which is connected between the center point of thesecond conduction path and the third connection. The center point of aconduction path is to be understood in this connection as the connectionbetween the switching element and the freewheeling diode within theconduction path. The center points of the two conduction paths areconnected to one another via a smaller inductor. Thus, the smallerinductor and the larger inductor are connected in series between thecenter point of the first conduction path and the third connection,while only the larger inductor is connected between the center point ofthe second conduction path and the third connection.

In one embodiment the two switching elements of the half-bridge are ofdifferent types. In this case, the switching element in the firstconduction path causes higher switching losses than the switchingelement in the second conduction path. By contrast, the switchingelement in the second conduction path has a body diode. The fact thatthe body diode acts as a freewheeling diode onto which the current iscommutated when the clocked switching element in the first conductionpath is opened is undesired, because this would lead to EMC problems,high switching losses and possibly even to destruction of the switchingelement in the second conduction path. A smaller inductor is arrangedbetween the center points of the conduction paths so that the currentinstead commutates onto the freewheeling diode arranged in the firstconduction path. It interacts with the larger inductor as inductivevoltage divider, with the result that a higher voltage is present acrossthe freewheeling diode in the first conduction path when the switchingelement in the first conduction path is closed than across the bodydiode of the switching element in the second conduction path. However,the smaller inductor does not have this protective function for theswitching element in the first conduction path if the clocked switchingelement in the second conduction path is open. In the case of the closedswitching element in the second conduction path and the open switchingelement in the first conduction path, no current flows via the smallerinductor and correspondingly no voltage drops across the small inductorin the case of subsequent interruption of the flow of current by openingthe switch in the second conduction path. In the case of thebidirectional converter according to the disclosure, the switchingelement in the first conduction path is not one with a body diode,however, onto which commutation of the current is to be prevented. Inthe case of the bidirectional converter according to the disclosure, thesecond direction between the pair of high voltage-side connections andthe pair of low voltage-side connections, in which the flow of energy iscaused by high-frequency clocking of the switching element in the secondconduction path with the lower switching losses, is a preferreddirection of the flow of energy between the pair of high voltage-sideconnections and the pair of low voltage-side connections.

In practice, in the case of the bidirectional converter according to thedisclosure, the switching element in the first conduction path ispreferably an IGBT, while the switching element in the second conductionpath is an FET (field-effect transistor) in one embodiment; this can be,for example, a JFET and, in particular, a MOSFET. In this case, the FETis connected to the first connection or the second connection such that,when it is operated in a clocked manner, power flows in a preferreddirection between the one and the other pair of connections of theconverter. By contrast, the IGBT is clocked in the first conduction pathin order to allow power to flow in the less preferred direction betweenthe two pairs of connections. If the preferred direction of the flow ofpower runs from the third and fourth connection to the first and secondconnection, that is to say from the pair of low voltage-side connectionsto the pair of high voltage-side connections, that is to say theconverter according to the disclosure operates as a step-up converter,the switching element in the second conduction path is connected to thesecond connection, which is connected to the fourth connection. Thesecond and the fourth connection are, like the first and the thirdconnection, alike, that is to say of the same polarity, with respect tothe voltages present.

However, if the preferred direction of the flow of energy runs from thefirst and second connection, that is to say from the pair of highvoltage-side connections, to the pair of low voltage-side connections,wherein the converter according to the disclosure operates as astep-down converter, the switching element in the second conduction pathis connected between the center point of the second conduction path andthe first connection, which is not directly connected to the thirdconnection.

Independently of the preferred direction of the flow of power or thepreferred function of the bidirectional converter according to thedisclosure as step-up converter or step-down converter, the switchingelement in the second conduction path is clocked in the second operatingmode of the half-bridge in order to realize the preferred flow of power.Conversely, the switching element in the first conduction path is alwaysclocked in the first operating mode of the half-bridge for the flow ofpower in the less preferred direction. Although, the switching element,which has no body diode and is, in particular, embodied as an IGBT,causes higher switching losses in the case of an identically large flowof current than the switching element in the second conduction path, inthe less preferred direction of the flow of power, only small currentsoccur and are rarer, with the result that the worse efficiency of theIGBT is hardly of any consequence. Compared to a half-bridge having twoconduction paths which are each populated with a MOSFET and aredecoupled between their center points by two small inductors, the higherswitching losses are more than compensated by the simpler constructionof the switching element in the first conduction path and only one smallinductor between the center points of the two conduction paths and hencein total only one smaller and one larger inductor for the entirehalf-bridge of the converter according to the disclosure. By contrast,the efficiency of the converter according to the disclosure in thepreferred direction of the power transfer is particularly high, withoutthere being risks to the body diode of the switching element in thesecond conduction path when the switching element in the firstconduction path is clocked.

During the preferred operation of the converter according to oneembodiment of the disclosure, only the larger inductor is energized. Theinductance of the smaller inductor can be kept very small in comparisonwith the inductance of the larger inductor, since it is only used forthe inductive voltage division during the less preferred operation ofthe converter according to the disclosure. During said less preferredoperation, the smaller inductor must allow the voltage across thefreewheeling diode in the first conduction path to break down only somuch higher than the voltage across the switching element with the bodydiode in the second conduction path that only the freewheeling diode inthe first conduction path becomes conducting. For this purpose, in oneembodiment it is sufficient for the inductance of the smaller inductorto be less than a tenth, generally even not more than a hundredth of theinductance of the larger inductor.

The fact that the switching element in the first conduction path has nobody diode does not rule out that a protective diode can be connected inparallel with the switching element. The protective diode has the sameforward direction between the first and second connection as thefreewheeling diode in the first conduction path. The protective diode isconfigured such that it only becomes conducting at a higher voltage thanthe freewheeling diode in the second conduction path and/or such that itis not damaged even by a larger current flowing through it. A protectivediode which is only small and has a significantly higher switch-onvoltage than that of the protective diode in the second conduction pathis less expensive.

It goes without saying that, in the case of the bidirectional converteraccording to the disclosure, smoothing capacitors can be connectedbetween the first and the second connection and/or between the third andthe fourth connection.

In the case of the converter according to the disclosure, in addition tothe one half-bridge described thus far, a further half-bridge can beprovided, wherein the two half-bridges are designed and arranged so asto be mirror-symmetrical about a common connection of the common secondconnection and the common fourth connection of the two half-bridges,except for the forward directions of their diodes and the blockingdirections of their switching elements. It goes without saying that themirror-symmetrical design of the two half-bridges does not relate to thepolarity of the connections and therefore neither to the forwarddirections of the diodes of the half-bridges nor to the blockingdirections of the switching elements of the half-bridges. For each ofthe two half-bridges, the converter according to the disclosure hasdedicated first and third connections, wherein the first and the thirdconnection of the one half-bridge and the first and the third connectionof the other half-bridge have reversed polarities.

The voltages of the first and second connections, between which the twohalf-bridges are connected, can then be, in particular, part voltages ofa divided intermediate circuit. Likewise, the voltages between the thirdand fourth connections of the half-bridges can be part voltages of afurther divided intermediate circuit. The center points of the twodivided intermediate circuits are in this case the common second andfourth connections which are directly connected to one another.

A reactive power-capable inverter according to the disclosure having abidirectional converter according to the disclosure can have acommutator connected to the third and fourth connection, and analternating power grid connectable to the other side of the commutator.In this embodiment, the preferred direction of the flow of power is fromthe first and second connection to the third and fourth connection, thatis to say the converter operates primarily as step-down converter. Inone embodiment the step-up converter function thereof is restricted tothe provision of feedback power in the periods in which current andvoltage have different mathematical signs during the provision ofreactive power.

It goes without saying that if two half-bridges which are designed andarranged so as to be mirror-symmetrical with respect to one another areprovided, the commutator is connected to the two third connections ofthe two half-bridges.

The converter operated as step-down converter forms current half-waveswhich are conducted to the alternating-current connections from thecommutator with alternating orientation.

In one embodiment the switches of the commutator are only clocked atsystem frequency and can therefore be designed as inexpensive IGBTswithout significant losses owing to switching losses.

Since complete feedback capability is usually not required forinverters, but, for example, only an operation at cos φ>0.8, the maximumsystem current does not regularly flow during the periods in whichcurrent and voltage have different mathematical signs. Hence, it ispossible, in principle, to clock the switching element in the firstconduction path more slowly than the switching element in the secondconduction path and hence to allow a greater ripple current withoutrisking magnetic saturation of the larger inductor. Thus, the higherswitching losses which are associated with clocking the switchingelement in the first conduction path can be reduced.

A reactive power-capable inverter according to the disclosure having abidirectional converter according to the disclosure can also be designedsuch that a further identically designed converter is connected on oneside between the first and second connections and on the other sidebetween fifth and sixth connections, wherein the fifth and sixthconnections correspond to the third and fourth connections of the firstconverter. An alternating power grid is connectable to the third andfifth connections of the two converters, which are connected via thelarger inductors to the center points of the second conduction paths ofthe two half-bridges of the two converters. The fourth and sixthconnections of the two converters are connected to one another. In thecase of this construction of the inverter, the two half-bridgesalternately supply a half-wave of the alternating current, while theother half-bridge is connected through in order to connect the thirdconnection directly to the fourth connection or the fifth connectiondirectly to the sixth connection. For the direct connection, a bypassswitch can be provided both between the third and the fourth connectionof the first converter and the fifth and the sixth connection of thefurther converter, which bypass switches bypass the half-bridge of therespective converter in half-waves. In one embodiment the bypassswitches are simple and inexpensive IGBTs.

Now referring in greater detail to the drawings, a converter 1 shown inFIG. 1 is connected between a DC-voltage intermediate circuit 2 with ahigher voltage U_high and a DC-voltage intermediate circuit 3 with alower voltage U_low. In this case, the converter 1 is connected via afirst connection 4 and a second connection 5 to the DC-voltageintermediate circuit 2 and via a third connection 6 and a fourthconnection 7 to the DC-voltage intermediate circuit 3. The fourthconnection 7 is directly connected to the second connection 5. The thirdconnection 6 is connected to a half-bridge 9 via a larger (first)inductor 8. The half-bridge 9 has two parallel conduction paths 10 and11. Each of the conduction paths 10 and 11 runs between the connections4 and 5, and in each of the conduction paths 10 and 11 a switchingelement 12 or 13 is connected in series with a freewheeling diode 14 or15. The center points 16 or 17 of the two conduction paths 10 and 11between the respective switching elements 12 and 13, and the respectivefreewheeling diodes 14 and 15 are connected to one another via a smaller(second) inductor 18, the inductance of which is only a fraction of theinductance of the larger (first) inductor, usually less than 1/10, ofteneven not more than 1/100. The center point 17 of the conduction path 11is connected to the larger (first) inductor 8. That is to say that theconnection 6 is connected via the larger (first) inductor 8 to thecenter point 17 and via the two inductors 8 and 18 to the center point16. The switching elements 12 and 13 and the freewheeling diodes 14 and15 in the conduction paths 10 and 11 of the half-bridge 9 are arrangeddiagonally. That is to say the switching element 12 in the conductionpath 10 is connected to the connection 4, while the switching element 13in the conduction path 11 is connected to the connection 5. The oppositesituation applies to the freewheeling diodes 14 and 15. In oneembodiment the switching element 12 in the conduction path 10 is an IGBT19, and the switching element 13 in the conduction path 11 is a MOSFET20 with a body diode 21. The forward direction of the body diode 21 runsparallel to the forward direction of the freewheeling diode 14 in theconduction path 10.

The design of the converter 1 according to FIG. 1 comprises the step-upconverter 22, which comprises the switching element 13 in the form ofthe MOSFET 20 and the freewheeling diode 15 of the conduction path 11and the larger (first) inductor 8, and is illustrated in FIG. 2. Duringoperation of the step-up converter by clocking the switching element 13,a flow of current 23 and, correspondingly, a flow of energy 24 occurs inthe direction from the DC-voltage intermediate circuit 3 with lowervoltage U_low to the DC-voltage intermediate circuit 2 with highervoltage U_high. The step-up converter 22 according to FIG. 2 in the caseof the converter 1 according to FIG. 1 is combined with a step-downconverter 25 according to FIG. 3, which step-down converter comprisesthe switching element 12 in the form of the IGBT 19 and the freewheelingdiode 14 of the conduction path 10 and the series connection composed ofthe smaller (second) inductor 18 and the larger (first) inductor 8.During operation of the step-down converter 25 by clocking the switchingelement 12, a flow of current 26 and a flow of energy 27 occur betweenthe DC-voltage intermediate circuits 2 and 3, the directions of whichare opposite to those of the flow of current 23 and the flow of energy24 in the case of the step-up converter according to FIG. 2.

In the case of the converter 1 according to FIG. 1, by selectivehigh-frequency clocking of one of the switching elements 12 and 13, thefunction of the step-down converter 25 according to FIG. 3 or of thestep-up converter 22 according to FIG. 2 can be realized. Thus, a flowof current 28 and a flow of energy 29 are possible in both directionsbetween the DC-voltage intermediate circuits 2 and 3. However, thereexists the danger that, when the switching element 12 is opened duringoperation of the converter 1 as step-down converter, the current doesnot commutate onto the freewheeling diode 14 provided for this purposebut rather onto the body diode 21 of the MOSFET 20 which is provided asswitching element 13 for the operation of the converter 1 as step-upconverter. This undesired commutation of the current onto the body diode21 is suppressed by the smaller (second) inductor 18 connected betweenthe center points 16 and 17 of the two conduction paths 10 and 11, whichsmaller inductor acts together with the larger (first) inductor 8 asinductive voltage divider. Thus, the voltage present across thefreewheeling diode 14 when the switching element 12 is opened duringoperation of the converter 1 as a step-down converter is significantlyhigher than the voltage present across the body diode 21.Correspondingly, the current commutates onto the freewheeling diode 14,as desired. During operation of the converter 1 as a step-up converterby clocking the switching element 13, the smaller (second) inductor 18has no voltage-dividing function. This is not necessary, however,because the IGBT 19 as switching element 12 has no body diode onto whichcommutation of the current must be prevented.

In the case of the converter 1 according to FIG. 1, the preferredoperation of that converter is as a step-up converter. Therefore, theswitching element 13 provided for the operation as a step-up converteris embodied as a MOSFET 20 which is more expensive in comparison withthe IGBT 19. In the case of high-frequency clocking in the region ofseveral 10 kHz, a MOSFET has lower switching losses than an IGBT.High-frequency clocking of the switching elements 12 and 13 is arequirement so that the larger (first) inductor 8 can be dimensioned soas to be small and hence light and inexpensive. During less preferredoperation of the converter 1 according to FIG. 1 as step-down converter,the somewhat higher switching losses of the IGBT 19 are tolerated, whichIGBT is less expensive than the MOSFET 20. The higher switching lossesoccur less often than the lower switching losses of the MOSFET 20.

In the case of the converter 1 according to FIG. 4, the sequence of theswitching elements 12 and 13 and the freewheeling diodes 14 and 15 isswapped in the two conduction paths 10 and 11 in comparison with FIG. 1.Furthermore, however, the switching element 12 in the conduction path10, the center point 16 of which is connected via both inductors 18 and8 to the connection 6, is the IGBT 19, while the switching element 13 inthe conduction path 11, the center point 17 of which is connected onlyvia the larger (first) inductor 8 to the connection 6, is the MOSFET 20.In this way, the converter 1 according to FIG. 4 is designed forpreferred operation as a step-down converter with clocking of theswitching element 13. Operation of the converter as a step-up converterwith clocking of the switching element 12 is less preferred. Duringpreferred operation of the two converters 1 according to FIGS. 1 and 4,the switching element 13 in the conduction path 11 is therefore alwaysclocked, while during less preferred operation the switching element 12in the conduction path 10 is clocked. During preferred operation, lowerswitching losses than in the less preferred operation always occur. Theconstruction of the converter 1 is particularly simple in each case. Thetwo converters 1 according to FIGS. 1 and 2 differ only in thearrangement of the switching elements 12 and 13 and the freewheelingdiodes 14 and 15 in the two conduction paths 10 and 11.

The fact that, in FIG. 4, a protective diode 30 is provided connected inparallel with the switching element 12 in the form of the IGBT 19 is nota fundamental differentiating feature of a converter 1 with preferredoperation as a step-down converter from a converter 1 with preferredoperation as a step-up converter. Instead, such a protective diode couldalso be provided in the case of the IGBT 19 according to FIG. 1. In anycase, it has a larger switch-on voltage than the freewheeling diode 15connected in parallel therewith.

In the case of the reactive power-capable inverter 31 according to thedisclosure and illustrated in FIG. 5, the converter 1 has theconstruction according to

FIG. 4, except for the protective diode 30 which is not present in thiscase, and is preferably operated as step-down converter by clocking theswitching element 13, while, by clocking the switching element 12, theconverter is also operable as step-up converter. During operation as astep-down converter, the converter forms half-waves from the currentflowing from the DC-voltage intermediate circuit 2. A photovoltaicgenerator 38 is connected via a step-up converter 51 to the intermediatecircuit capacitor 48 of the DC-voltage intermediate circuit 2. Thestep-up converter 51 has a step-up converter inductor 52, a step-upconverter switch 53 and a step-up converter diode 54 in the typicalarrangement according to FIG. 2. The DC-voltage intermediate circuit 3has a smaller intermediate circuit capacitor 49 which is used only forsmoothing the switch ripple in the case of the half-waves of thecurrent. A commutator 39 connected downstream of the DC-voltageintermediate circuit 3 reverses the polarity of every second half-waveof the current and thus feeds an alternating current into a connectedalternating power grid 40. The commutator is formed from switchingelements 41 and freewheeling diodes 42 in a known manner. The switchingelements 41 are embodied as IGBTs 43. Owing to the possibility ofoperating the converter 1 as a step-up converter, the inverter 31according to FIG. 5 is also completely reactive power-capable, byenabling a flow of current from the alternating power grid 40 to theDC-voltage intermediate circuit 2.

In the case of the reactive power-capable inverter 31 according to thedisclosure and illustrated in FIG. 6, a converter 1 has two subcircuits32 and 33 each of which corresponds to a converter 1 according to FIG.4, except for the protective diode 30 which is not present in this case.Therefore, the parts of the subcircuit 32 are provided here with thesame reference signs as the parts of the converter in FIG. 4, while theparts of the subcircuit 33 are provided with same reference signs whichare extended by a prime mark “′”. The two subcircuits 32 and 33 aredesigned and arranged so as to be mirror-symmetrical with respect totheir common connections 5 and 7 and the common connection thereof,except for the forward directions of their freewheeling diodes 14 and 15and 14′ and 15′ and the blocking directions of their switching elements12 and 13 and 12′ and 13′. Each subcircuit 32 and 33 therefore has itsown half-bridge 9 or 9′. The two half-bridges 9 and 9′ are connected onthe input side to two parts of a divided DC-voltage intermediate circuit34, that is to say in this case to one of two series-connectedcapacitors 35 and 35′. On the output side, the subcircuits 32 and 33 areconnected to two parts of an output-side DC-voltage intermediate circuit36, which in this case are represented by series-connected capacitors 37and 37′. The two subcircuits 32 and 33 are combined to form a converter1, which is preferably operated as a step-down converter by coordinatedclocking of the switching elements 13 and 13′, while it is also operableas a step-up converter by coordinated clocking of the switching elements12 and 12′. During operation as a step-down converter, the converterforms half-waves from a current which is generated by a photovoltaicgenerator 38, which is directly connected to the DC-voltage intermediatecircuit 34 in this case. A commutator 39 connected downstream of thedivided DC-voltage intermediate circuit 36, having the same constructionand the same function as in FIG. 5, reverses the polarity of everysecond half-wave and thus feeds an alternating current into a connectedalternating power grid 40. Owing to the possibility of operating theconverter 1 as a step-up converter, the inverter 31 according to FIG. 6is also completely reactive power-capable, by enabling a flow of currentfrom the system to the divided DC-voltage intermediate circuit 34.

FIG. 7 illustrates a further reactive power-capable inverter 44according to the disclosure. In this case, two half-bridges 9 and 9′having fundamentally the same construction as in FIG. 4, except forprotective diodes, which are not present in this case, are connected inparallel with one another to the connections 4 and 5. The otherconnections of the one half-bridge 9 are the third connection 6 and thefourth connection 7, while the connections of the other half-bridge 9″are a fifth connection 50 and a sixth connection 60. The further partsof the one half-bridge 9 are provided with the same reference signs inthis case as the parts of the converter 1 in FIG. 4, while the furtherparts of the other half-bridge 9″ are provided with same reference signsextended by a double prime mark “″”. The connections 6 and 50, whichcorrespond to one another, of the two half-bridges 9 are connected tothe alternating power grid 40, while their connections 7 and 60 areconnected to one another. The inverter 44 thus dispenses with a specificcommutator. Instead, the half-bridges 9 and 9″ are operated alternatelyin half-waves as step-down converters in order to form a half-wave ofthe current to be fed into the alternating power grid 40. Thehalf-bridge 9 or 9″ which is respectively inactive in this case isconnected through, by the switching element 12 or 12″ thereof in theform of the IGBT 19 or 19″ being permanently closed. Alternatively, acapacitor 45 or 45″ forming the respective DC-voltage intermediatecircuit 3 or 3″ can be bypassed by an additional bypass switch 46 or 46″in the form of an IGBT 47 or 47″ in order to avoid ohmic losses in thecase of a flow of current through the inductors 8 and 18 and 8″ and 18″.The inverter 44 is reactive power-capable, too, because each of itshalf-bridges 9 or 9″ and the converter 1 or 1″ provided thereby isbidirectional and thus can be operated as step-up converter in order toallow a current to flow from the system into the DC-voltage intermediatecircuit 2 at higher voltage.

1. A bidirectional converter, comprising: a first connection, a secondconnection, a third connection and a fourth connection, wherein thefirst and the second connections form a pair of high voltage-sideconnections between which a first voltage is present during operation ofthe converter, and wherein the third and the fourth connections form apair of low voltage-side connections between which a second voltage ispresent during operation of the converter, wherein the first voltage isat least as high as the second voltage, a first inductor and a secondinductor, a half-bridge comprising two switching elements and twofreewheeling diodes, wherein the half-bridge is divided into a firstconduction path and a second conduction path, which are connectedtogether in parallel between the first connection and the secondconnection, wherein, in each of the first and second conduction paths,one of the switching elements and one of the freewheeling diodes areconnected in series, and center points of the first and secondconduction paths are connected to one another via the second inductor,wherein the switching element in the first conduction path is connectedto another connection of the pair of high voltage-side connections thanthe switching element in the second conduction path, and thefreewheeling diode in the first conduction path is connected to anotherconnection of the pair of high voltage-side connections than thefreewheeling diode in the second conduction path, wherein thehalf-bridge has a first operating mode, in which the switching elementin the first conduction path is clocked at a high frequency in order tocause a flow of energy in a first direction between the pair of highvoltage-side connections and the pair of low voltage-side connections,wherein the half-bridge has a second operating mode, in which it drivesthe switching element in the second conduction path at a high frequencyin order to cause a flow of energy in a second direction which isopposite to the first direction between the pair of high voltage-sideconnections and the pair of low voltage-side connections, and whereinthe switching element in the second conduction path has a body diode,and wherein the first inductor is connected between the center point ofthe second conduction path and the third connection, wherein the secondinductor and the first inductor are connected in series between thecenter point of the first conduction path and the third connection,while only the first inductor is connected between the center point ofthe second conduction path and the third connection, and wherein the twoswitching elements are of different types, wherein the switching elementin the first conduction path does not have a body diode and causeshigher switching losses than the switching element in the secondconduction path.
 2. The converter of claim 1, wherein the seconddirection is a preferred direction of the flow of energy between thepair of high voltage-side connections and the pair of low voltage-sideconnections.
 3. The converter of claim 1, wherein the switching elementin the first conduction path is an insulated gate bipolar transistor(IGBT) and the switching element in the second conduction path is afield effect transistor (FET).
 4. The converter of claim 1, wherein apreferred direction of the flow of energy runs from the pair of highvoltage-side connections to the pair of low voltage-side connections. 5.The converter of claim 4, wherein the switching element in the secondconduction path is connected between the center point of the secondconduction path and the first connection.
 6. The converter of claim 1,wherein the switching element in the first conduction path has aprotective diode connected in parallel therewith, which protective diodehas the same forward direction between the first connection and thesecond connection as the freewheeling diode in the first conductionpath.
 7. The converter of claim 1, further comprising a smoothingcapacitor connected between the first connection and the secondconnection and/or the third connection and the fourth connection.
 8. Theconverter of claim 1, further comprising a further half-bridge, whereinthe two half-bridges are designed and arranged so as to bemirror-symmetrical about a common connection of the common secondconnection and the common fourth connection of the two half-bridgesexcept for the forward directions of their diodes and the blockingdirections of their switching elements.
 9. An inverter including abidirectional converter, the converter comprising: a first connection, asecond connection, a third connection and a fourth connection, whereinthe first and the second connections form a pair of high voltage-sideconnections between which a first voltage is present during operation ofthe converter, and wherein the third and the fourth connections form apair of low voltage-side connections between which a second voltage ispresent during operation of the converter, wherein the first voltage isat least as high as the second voltage, a first inductor and a secondinductor, a half-bridge comprising two switching elements and twofreewheeling diodes, wherein the half-bridge is divided into a firstconduction path and a second conduction path, which are connectedtogether in parallel between the first connection and the secondconnection, wherein, in each of the first and second conduction paths,one of the switching elements and one of the freewheeling diodes areconnected in series, and center points of the first and secondconduction paths are connected to one another via the second inductor,wherein the switching element in the first conduction path is connectedto another connection of the pair of high voltage-side connections thanthe switching element in the second conduction path, and thefreewheeling diode in the first conduction path is connected to anotherconnection of the pair of high voltage-side connections than thefreewheeling diode in the second conduction path, wherein thehalf-bridge has a first operating mode, in which the switching elementin the first conduction path is clocked at a high frequency in order tocause a flow of energy in a first direction between the pair of highvoltage-side connections and the pair of low voltage-side connections,wherein the half-bridge has a second operating mode, in which it drivesthe switching element in the second conduction path at a high frequencyin order to cause a flow of energy in a second direction which isopposite to the first direction between the pair of high voltage-sideconnections and the pair of low voltage-side connections, and whereinthe switching element in the second conduction path has a body diode,and wherein the first inductor is connected between the center point ofthe second conduction path and the third connection, wherein the secondinductor and the first inductor are connected in series between thecenter point of the first conduction path and the third connection,while only the first inductor is connected between the center point ofthe second conduction path and the third connection, and wherein the twoswitching elements are of different types, wherein the switching elementin the first conduction path does not have a body diode and causeshigher switching losses than the switching element in the secondconduction path, and the inverter further including a commutator, oneside of the commutator being connected to the third connection and thefourth connection of the half-bridge, and the other side of thecommutator being configured to be connected to an alternating powergrid.
 10. The inverter of claim 9, wherein the switching element in thesecond conduction path is connected between the center point of thesecond conduction path and the first connection.
 11. The inverter ofclaim 9, wherein the commutator has IGBTs as switching elements.
 12. Aninverter including a bidirectional converter, the converter comprising:a first connection, a second connection, a third connection and a fourthconnection, wherein the first and the second connections form a pair ofhigh voltage-side connections between which a first voltage is presentduring operation of the converter, and wherein the third and the fourthconnections form a pair of low voltage-side connections between which asecond voltage is present during operation of the converter, wherein thefirst voltage is at least as high as the second voltage, a firstinductor and a second inductor, a half-bridge comprising two switchingelements and two freewheeling diodes, wherein the half-bridge is dividedinto a first conduction path and a second conduction path, which areconnected together in parallel between the first connection and thesecond connection, wherein, in each of the first and second conductionpaths, one of the switching elements and one of the freewheeling diodesare connected in series, and center points of the first and secondconduction paths are connected to one another via a smaller inductor,wherein the switching element in the first conduction path is connectedto another connection of the pair of high voltage-side connections thanthe switching element in the second conduction path, and thefreewheeling diode in the first conduction path is connected to anotherconnection of the pair of high voltage-side connections than thefreewheeling diode in the second conduction path, wherein thehalf-bridge has a first operating mode, in which the switching elementin the first conduction path is clocked at a high frequency in order tocause a flow of energy in a first direction between the pair of highvoltage-side connections and the pair of low voltage-side connections,wherein the half-bridge has a second operating mode, in which it drivesthe switching element in the second conduction path at a high frequencyin order to cause a flow of energy in a second direction which isopposite to the first direction between the pair of high voltage-sideconnections and the pair of low voltage-side connections, and whereinthe switching element in the second conduction path has a body diode,and wherein the first inductor is connected between the center point ofthe second conduction path and the third connection, wherein the secondinductor and the first inductor are connected in series between thecenter point of the first conduction path and the third connection,while only the first inductor is connected between the center point ofthe second conduction path and the third connection, and wherein the twoswitching elements are of different types, wherein the switching elementin the first conduction path does not have a body diode and causeshigher switching losses than the switching element in the secondconduction path, and the further converter comprising a furtherhalf-bridge, wherein the half-bridge and the further half-bridge aredesigned and arranged so as to be mirror-symmetrical about a commonconnection of the common second connection and the common fourthconnection of the two half-bridges except for the forward directions oftheir diodes and the blocking directions of their switching elements,and the inverter further including commutator, one side of thecommutator being connected to the third connections of the half-bridgeand the further half-bridge, and the other side of the commutator beingconfigured to be connected to an alternating power grid.
 13. Theinverter of claim 12, wherein the commutator has IGBTs as switchingelements.
 14. An inverter including a bidirectional converter, theconverter comprising: a first connection, a second connection, a thirdconnection and a fourth connection, wherein the first and the secondconnections form a pair of high voltage-side connections between which afirst voltage is present during operation of the converter, and whereinthe third and the fourth connections form a pair of low voltage-sideconnections between which a second voltage is present during operationof the converter, wherein the first voltage is at least as high as thesecond voltage, a first inductor and a second inductor, a half-bridgecomprising two switching elements and two freewheeling diodes, whereinthe half-bridge is divided into a first conduction path and a secondconduction path, which are connected together in parallel between thefirst connection and the second connection, wherein, in each of thefirst and second conduction paths, one of the switching elements and oneof the freewheeling diodes are connected in series, and center points ofthe first and second conduction paths are connected to one another viathe second inductor, wherein the switching element in the firstconduction path is connected to another connection of the pair of highvoltage-side connections than the switching element in the secondconduction path, and the freewheeling diode in the first conduction pathis connected to another connection of the pair of high voltage-sideconnections than the freewheeling diode in the second conduction path,wherein the half-bridge has a first operating mode, in which theswitching element in the first conduction path is clocked at a highfrequency in order to cause a flow of energy in a first directionbetween the pair of high voltage-side connections and the pair of lowvoltage-side connections, wherein the half-bridge has a second operatingmode, in which it drives the switching element in the second conductionpath at a high frequency in order to cause a flow of energy in a seconddirection which is opposite to the first direction between the pair ofhigh voltage-side connections and the pair of low voltage-sideconnections, and wherein the switching element in the second conductionpath has a body diode, and wherein the first inductor is connectedbetween the center point of the second conduction path and the thirdconnection, wherein the second inductor and the first inductor areconnected in series between the center point of the first conductionpath and the third connection, while only the first inductor isconnected between the center point of the second conduction path and thethird connection, and wherein the two switching elements are ofdifferent types, wherein the switching element in the first conductionpath does not have a body diode and causes higher switching losses thanthe switching element in the second conduction path, and the inverterfurther including a further bidirectional converter, identical to theconverter and provided between the first connection and the secondconnection on one side, and a fifth connection and a sixth connection onanother side, wherein an alternating power grid is connectable to thethird connection and the fifth connection, which are connected via thelarger inductors of the converter and the further converter to thecenter points of the second conduction paths of the two half-bridges ofthe converter and the further converter, and wherein the fourthconnection and the sixth connection are connected to one another. 15.The inverter of claim 14, further comprising a bypass switch providedbetween the third connection and the fourth connection, and a furtherbypass switch provided between the fifth connection and the sixthconnection.
 16. The inverter of claim 15, wherein the bypass switch andthe further bypass switch are IGBTs.